1. Field of the Invention
The invention pertains to a method of forming images from metastable aggregated-metal colloids, and materials prepared therefrom. In particular, a coating of an aggregated-metal in a polymer matrix may be caused to undergo a color change by the application of thermal energy, to create a permanent image. Selective application of thermal energy can be used to provide a color image on a color differentiated background.
2. Background of the Prior Art
The preparation of stable colloids or hydrosols of metallic silver and gold has been known for some time. One method of preparation of the same is set forth in Frens, G. and Overbeek, J., Kolloid Z. Z. Polym., 233, 922 (1969). Such colloids are characteristically colored. Primary colloids of spherical silver particles, for example, in which the silver is dispersed as separated particles of nonaggregated spheres of silver, are yellow. The use of such colloids in photographic systems as blue light filters is known.
It is also known that the addition of an electrolyte to the colloid, per se, will induce the primary, non-aggregated particles to aggregate as shown by the Frens and Overbeek article referenced above. As the aggregate adds more and more of the primary particles, the observed color of the colloid changes. In the case of silver, the color changes from yellow to red, green, violet, blue, brown and gray. Eventually, if aggregation continues, the aggregates fall out of suspension and precipitate.
In U.S. Pat. application Ser. No. 344,949, Gilmour et. al. and U.S. Pat. application Ser. No. 344,950, Shuman, both filed Apr. 28, 1989, metastable Group Ib metal colloids, their preparation, and use in thermally forming stable images, is disclosed. In those applications, non-spherical silver or related Group Ib metal colloids are converted to the spherical, yellow form by application of thermal energy. In those applications, the metal particles remain nonaggregated, and the background color changes to the yellow of the spherical particle. This change is induced by a thermal recrystallization, leading to the stable spherical colloidal form, and a different color. The accompanying color change is not related to any aggregation process.
Defensive Publication T900,010 relates to blue colloidal silver and its use in obtaining an image by contacting the blue silver with halide ions to cause the blue silver to recrystallize to a yellow form. No mention is made in this publication of the use of thermal energy to cause imaging in a metastable aggregated-metal colloid system.
Accordingly, it remains a goal of those of skill in the art to provide methods for forming color-differentiated stable images, particularly for use in conjunction with optical reading devices. Ideally, the material should be stable over time, and relatively insensitive to ambient conditions, but, upon conversion, should give a stable, highly-resolved image on a differentiated background.